The present invention relates to a microstrip antenna used as a built-in antenna of a portable telephone or mobile terminal for example.
A xcex/2 patch antenna is a typical microstrip antenna to be built in a portable telephone or a mobile terminal such as a GPS. In this case, xcex denotes a wavelength in a frequency used.
This antenna is mainly constituted of a dielectric substrate having a rectangular or circular radiating conductor (patch conductor) with a side length or a diameter of approximately xcex/2 on one face and having a ground plate conductor on the other face.
It has been recently requested to further downsize the portable telephone and mobile terminal and thereby, it is requested to further downsize a built-in type patch antenna. A dielectric substrate with a high dielectric constant is typically used to physically downsize the patch antenna with the above-mentioned patch conductor dimension of approximately xcex/2.
However, the relative dielectric constant of a dielectric material having a low temperature coefficient suitable for a high frequency is up to xcex5r of approximately 110 and therefore, it is limited to downsize an antenna by raising the dielectric constant of the dielectric material. Since a dielectric material becomes more expensive by raising its dielectric constant, the cost for fabricating a microstrip antenna will increase if such raised dielectric constant material is used.
Japanese patent publication No. 05152830 A (U.S. Pat. No. 2,826,224) discloses, as a known art for downsizing a microstrip antenna without raising the dielectric constant of the dielectric material, to produce two resonant modes orthogonal to each other and having phases different from each other by forming a degenerate separation element, to form a power-supply point in a straight-line direction orthogonal to the direction of the resonant mode at xc2x145xc2x0, and to form notches at the both ends in the straight-line direction of the radiating conductor. By forming such notches, it is possible to equivalently increase electric lengths of two resonant modes, and lower a resonance frequency. Therefore, it is possible to downsize the antenna element to a certain extent.
Japanese patent publication No. 06276015 A discloses, as a known art of a microstrip antenna, that two crossing slots with different lengths from each other are formed as a degenerate separation element in a radiating conductor and that notches or stubs are formed at the outer edge of the radiating conductor in order to adjust the inductance component of the radiating conductor.
Japanese patent publication No. 09326628 A discloses, as another known art of a microstrip antenna, that two resonance characteristics for generating two modes with different route lengths from each other are obtained by forming a crossed cutout with two arm lengths different from each other on a square radiating plate so that these symmetry axes coincide with two diagonal lines of the plate, respectively.
However, according to the known art disclosed in Japanese patent publication No. 05152830 A (U.S. Pat. No. 2,826,224), because the notches are formed only the both ends of the radiating conductor in the direction coinciding with the power-supply point of the conductor and a current-route width is not changed at the central portion of the radiating conductor corresponding to an antinode of current flowing under resonance, it cannot be expected to greatly reduce a resonance frequency. Furthermore, since a capacitance with respect to ground is reduced by forming the notches at the both ends of the radiating conductor corresponding to antinodes of voltage under resonance, it cannot be also expected to greatly reduce the resonance frequency. Therefore, it is difficult to extremely downsize the microstrip antenna.
Although Japanese patent publication No. 06276015 A discloses to form two crossing slots having different lengths from each other as a degenerate separation element, it is silent for downsizing an antenna element. In this disclosed art furthermore, since notches or stubs are formed at the outer edge of the radiating conductor, it is impossible to effectively use the limited surface area of a dielectric substrate for improving the radiation efficiency.
In addition, although Japanese patent publication No. 09326628 A discloses that two resonance characteristics are obtained by forming a crossed cutout with two arm lengths different from each other so that symmetry axes coincide with diagonal lines of a radiation plate, it is silent for downsizing an antenna element at all. Moreover, because the position of the power-supply point is present on a vertical line passing through the center of a side, it is very difficult to mount an antenna element when it is downsized and its terminal interval is decreased.
It is therefore an object of the present invention to provide a microstrip antenna, whereby further downsizing can be expected.
Another object of the present invention is to provide a microstrip antenna, whereby its radiation efficiency can be improved by effectively using the limited surface area of a dielectric substrate.
A further object of the present invention is to provide a microstrip antenna, whereby a power-supply point is located at an easily-mounting position.
According to the present invention, a microstrip antenna includes a rectangular dielectric substrate, a ground plate conductor formed on one surface of the dielectric substrate, a rectangular radiating conductor formed on the other surface of the dielectric substrate, a crossed slot formed in the radiating conductor and provided with two arms extended along orthogonal sides of the radiating conductor, the two arms having lengths different from each other, and at least one power-supply point formed on a diagonal line of the radiating conductor or an extension line of the diagonal line but different from a center of the radiating conductor. The length of at least one of the arms is equal to or more than a value obtained by subtracting a four times value of a thickness of the dielectric substrate from a length of a side of the radiating conductor along the arm.
Thus, according to the present invention, the length of at least one of the two arms of the crossed slot, parallel with orthogonal sides of the radiating conductor is set so as to be equal to or more than a value obtained by subtracting a four times value of the thickness of the dielectric substrate from the length of the side of the radiating conductor in that direction. That is, if it is assumed that a central point of each arm is located at the center of the radiating conductor, the distance between the top end of at least one arm of the slot and outer edge of the radiating conductor is set so that the distance becomes equal to or less than a double value of the thickness of the dielectric substrate. Each region between the top end of the arm or slot and the outer edge of the radiating conductor locates at the antinode of current in a current route under resonance. Therefore, by decreasing the width of the region of the current route, magnetic field is concentrated on the region to increase the inductance at that region, and the area of the region decreases to lower the capacitance at the region. Thus, by making a region with a low potential more inductive, the resonance frequency lowers resulting that dimensions of a microstrip antenna are further decreased.
Particularly, according to the present invention, the distance between the top end of at least one arm of the slot and the outer edge of the radiating conductor, in other words, the width of a current route serving as an antinode of current in the current route under resonance is set so as to be equal to or less than a double value of the thickness of the dielectric substrate. Therefore, a resonance frequency is greatly lowered and as a result, it is possible to further downsize an antenna.
Furthermore, since at least one power-supply point is located on a diagonal line or an extension line of the diagonal line except a center of the radiating conductor and located at a corner of the radiating conductor, it is possible to easily perform wiring and mounting for power supply.
It is preferred that the length of each arm of the slot is equal to or more than a value obtained by subtracting a four times value of a thickness of the dielectric substrate from a length of a side of the radiating conductor along the arm.
It is also preferred that ends of the slot are rounded. By rounding the ends, it is prevented that current is concentrated on a part of each end and a conductor loss increases. That is, the flow of the current at the end becomes smooth and it is possible to reduce the conductor loss without increasing a pattern in size and therefore, it is possible to improve the Q due to the conductor loss.
It is preferred that at least one cutout or stub is formed at a crossing portion of the slot. By forming at least one cutout or stub for adjusting impedance characteristic and frequency characteristic on the slot and forming the radiating conductor as large as possible in the limited surface area of the dielectric substrate, it is possible to improve the area-utilization rate and radiation efficiency of the antenna. In this case, preferably at least one cutout or stub is formed on a diagonal line of the radiating conductor.
It is also preferred that the radiating conductor has a square shape and the arms of the slot tilt by xc2x145xc2x0 from a diagonal line on which the at least one power-supply point is present.
It is preferred that the antenna further includes an electrostatic coupling pattern constituted by cutting out a part of the radiating conductor to connect the at least one power-supply point with the radiating conductor. Since the electrostatic coupling pattern is formed by cutting out a part of the radiating conductor and at least one power-supply point is formed, it is possible to further improve the utilization efficiency of the radiating conductor.
It is also preferred that a thickness of the dielectric substrate is equal to or less than a xc2xc wavelength of a frequency used.
It is preferred that a length of a side of the dielectric substrate is equal to or less than a value obtained by adding a thickness of the dielectric substrate to a length of a side of the radiating conductor along the side of the dielectric substrate. In general, it is estimated that a side-fringing electric field becomes weaker as further separating from the outer edge of the radiating conductor and that the intensity of the electric field is decreased to approximately xc2xd at a position a half thickness of the dielectric substrate separate from the substrate. To effectively use the surface of a dielectric substrate, it is preferable to form the radiating conductor up to the outer edge of the dielectric substrate. In this case, however, most side-fringing electric field leaks to the outside of the substrate. Therefore, the distance between the outer edge of the dielectric substrate and that of the radiating conductor is set so as to be equal to or less than xc2xd of the thickness of the dielectric substrate by considering the end capacity effect and effective use of the dielectric substrate surface.
It is preferred that two power-supply points are provided at two positions that are point-symmetric to a center of the radiating conductor, respectively. Thereby, it is possible to directly connect the power-supply points of the antenna to an active circuit such as a differential amplifier and directly supply a signal having a phase difference of 180xc2x0.